The present invention is a method and apparatus for determining remaining basicity of a used cylinder lubricant from an open, also referred to as an all-loss, lubrication system. The present invention has benefit in providing a near real-time basicity estimate of the used lubricant from an open power-cylinder lubricating system of a slow-speed, two-stroke diesel engine of crosshead construction.
Slow-speed, two-stroke diesel engines of crosshead construction are used in marine propulsion, stationary power or other applications because of their efficient, high power output. Unlike a smaller four-stroke engine with a single closed lubricating system that continuously circulates lubricant from a reservoir, often referred to as a sump, to the engine components and back to the reservoir, the crosshead two-stroke engine has two lubricating systems, a closed recirculating system that lubricates and protect the surfaces in the crankcase, and an open system that applies a cylinder lubricant only once to lubricate and protect the power cylinder walls and pistons before the used lubricant is removed from the engine and discarded.
A major reason for using an open cylinder lubricating system is that sulfur content of fuel for slow-speed crosshead engines is typically in the range of 1.5 to 5.0%. This compares to fuel sulfur content of typically less than 0.05% for most medium and high speed diesel engines. Since sulfur reacts to form acids, especially during the combustion process, the higher sulfur content fuel results in higher acid content in the engine cylinders. If the acids formed during combustion are not effectively neutralized, they can attack engine surfaces to cause reduced engine performance and life. Lubricant used on internal-combustion-engine cylinder walls should, as one of is functionalities, neutralize acids. This neutralizing functionality is achieved by formulating the lubricant to have high alkalinity, that is, to be highly basic. A lubricants' alkalinity is described by a Total Base Number (TBN), often referred to as the lubricant's Base Number (BN), which is a measure of the amount of acid that a mass of lubricant can neutralized. TBN is reported in terms of milligrams of potassium hydroxide per gram (mg KOH/g) and is, in general, determined by either ASTM D2896 or D4739 titrations methods. The sulfur content of typical four-stroke diesel fuel is sufficiently low so that a sufficiently high TBN engine lubricant can be formulated to allow a relatively long useful life in a closed lubricating system. Lubricant useful life ends and the lubricant must be replaced when the remaining TBN reaches a limit, typically set by the engine manufacturer, below which the lubricant no longer provides the desired acid neutralization function. With the high sulfur content of slow-speed crosshead diesel engine fuel, a lubricant has not been formulated to have a long service life in a closed lubricating system; hence an open lubricating system should be used.
To provide desired engine performance and life at an acceptable cost, the formulation and flow rate of a lubricant in an open cylinder lubricating system needs to be optimized. The lubricant flow rate should be selected to be sufficiently high for a particular engine operating condition to provide a lubricating film to minimize friction between piston rings and cylinder walls, and the lubricant must be formulated so that at a selected flow rate and operating condition and at a given fuel sulfur content, there is an appropriate basicity to adequately neutralize acids entering the lubricant from the combustion process without having excess basicity that can cause piston ring deposits and ultimately ring and cylinder wall wear. Appropriate basicity is typically determined by the remaining TBN of the used cylinder lubricant that is removed from a cylinder.
Traditionally, the optimization of lubricant flow rate and formulation is done by selecting a lubricant with an appropriate TBN for an expected range of fuel sulfur contents and varying lubricant flow rate to the cylinders based on a table or formula that uses the actual sulfur content reported by the fuel supplier, the operating condition/state of the engine, and historic off-line TBN analyses of used cylinder lubricants. The TBN analyses are typically laboratory methods performed on samples removed from individual cylinders for known fuel sulfur content and operating conditions. A problem with this traditional optimization method is that the current flow rate decision is made based on historic TBN data and not real-time or near real-time data. Hence, flow rates are, in general, conservatively set higher than needed so as to protect against, for example, transient or incorrectly reported fuel sulfur content. While a too high flow rate is better than the consequences of having a too low flow rate, a too high flow rate results in added lubricant costs and can potentially lead to piston ring deposits.
To minimize both risks and costs, engine operators desire to know the remaining TBN in the used cylinder lubricant in either real-time or near real-time to set an appropriate lubricant flow rate. Methods that use acid reagents to measure or estimate TBN, such as the ASTM methods, are too complex in most engine applications to be accurately and/or quickly performed. Methods that use Infrared (IR) Spectroscopy to measure one or more oxidation peaks of the used lubricant, for example the method described in Reischman et al. U.S. Patent Application 2003/0164451, currently estimate TBN by near real-time methods. An issue with IR TBN sensors is that due to the relatively opaque nature of used cylinder lubricant the sensor must have a relatively narrow gap if a transmission technique is used or must use an Attenuated Total Reflectance (ATR) technique. A narrow gap of a transmission IR sensor can get blocked over long periods with the relatively high viscosity used lubricant, and a surface of an Internal Reflection Element (IRE) of an ATR IR sensor can get coated over long periods with the relatively high viscosity, surface active used lubricant. When used in situ for essentially real-time TBN estimates, IR sensors typically require periodic cleaning to assure accurate long-term results.
Another issue when determining the remaining TBN of a used cylinder lubricant is that the used lubricant can be contaminated by the system lubricant in the closed lubrication system that lubricates the engine's crankcase components. The crosshead construction of two-stroke engines has a diaphragm and stuffing boxes separating the power cylinders from the crankcase to prevent combustion by-products and cylinder lubricant from entering the crankcase and conversely to prevent crankcase lubricant from entering the cylinder and mixing with the used cylinder lubricant before removal from the engine. The diaphragm and stuffing boxes, however, are typically not 100% efficient and, in general, the efficiency decreases with engine use such that the lubricant removed from the power cylinders may be a mixture of used cylinder lubricant and crankcase lubricant with the ratio of the two lubricants varying with engine operating conditions. Hence, the measured TBN may not accurately represent the TBN of the used cylinder lubricant.
Therefore, there remains a need for a reliable, accurate method and apparatus for real-time or near real-time estimation of used cylinder-lubricant remaining TBN over long periods of use. Accordingly the present invention is a method that meets that need.